1. Field of the Invention
The present invention relates to a cross-coupled trisection filter, with inductance and capacitance devices, thereby reducing its physical size and increasing the production yield.
2. Description of Related Art
According to the filter design specification, if the degree of the resonator is increased, the selectivity of the frequency band is increased. However, this is accompanied with bandpass attenuation C and an increase in physical size. Refer to FIG. 1 for a prototype of a cascade trisection bandpass filter. As shown in FIG. 1, any cascade trisection bandpass filter generally provides asymmetric frequency response. Conventional bandpass filters with asymmetric response are further described in xe2x80x9cMicrostrip Cross-coupled trisection bandpass filters with asymmetric frequency charactersxe2x80x9d by J. -S. Hong and M. J. Lancaster, as shown in FIG. 2a, and in xe2x80x9cMicrostrip Cascade Trisection Filterxe2x80x9d by Chu-Chen Yang and Chin-Yang Chang, as shown in FIG. 2b. The resonators R1a, R2a, and R3a in FIG. 2a are construed on a substrate sun, wherein the resonator R1a has an input port IN and the resonator R3a has an output port OUT. The resonators R1b, R2b, R3b, R4b, and R5b in FIG. 2b are construed on a substrate (not shown), wherein the resonator R5a has an input port P1 and the resonator R3a has an output port P2. As shown in FIG. 2a, the 3-pole filter structure is composed of three xcex/2-line open-loop resonators R1a, R2a, R3a on one side of the dielectric substrate SUB with a ground plane on the other side. The cross coupling between resonators R1a and R3a exists because of their proximity. An attenuation pole of finite frequency exists on the high side of the pass band due to the cross-coupling. As shown in FIG. 2b, the 5-pole filter with two xcex/2-line open-loop resonators and three hairpin resonators has mixed (electric and magnetic) couplings between resonators R1b and R2b and between resonators R2b and R3b, the mixed couplings between resonators R3b and R4b and between resonators R4b and R5b. The lower attenuation pole is due to the nonadjacent magnetic coupling between resonators R1b and R3b, and the upper attenuation pole is due to the nonadjacent electric coupling between resonators R3b and R5b. Thus, both FIGS. 2a and 2b can achieve a higher selectivity without increasing the degree of poles, i.e. the number of resonators. However, such a structure exhibits increased size and easily suffers spurious effect on odd frequencies of the band pass (see the appendix A), so the required level of filtration is not achieved.
Accordingly, an object of the invention is to provide a filtering structure, which adds a serial capacitance device into each resonator of the filter in FIG. 1 to reduce the filter size.
Another object of the invention is to provide a small size cross-coupled trisection filtering structure, which uses the semi-lumped LC resonator to avoid the spurious effect and also keep the attenuation pole on the high frequency during the band pass.
Another object of the invention is to provide a small size cross-coupled trisection filtering structure, which only couples to the high impedance transmission portion of the resonators, thereby fitting a multilayer and easily adjusting the frequency of an attenuation pole by changing the high impedance transmission distance of the first and third poles without changing the bandpass characteristics.
The invention provides a small size cross-coupled trisection filter structure, including a first resonance unit; a second resonance unit; and a third resonance unit. Each of the units includes an inductance device, e.g. a transmission line, and a capacitance device, e.g. a capacitor, wherein the high impedance transmission portions of two of the units are coplanar and one has an input while the other has an output.